I. Field of the Invention
The present invention relates to a method for forming a monocrystalline semiconductor film on an insulating film and, more particularly, to a method for forming a monocrystalline semiconductor thin film which has a crystal lattice continuous to that of a monocrystalline substrate and which is formed on an insulating film formed on the monocrystalline substrate.
II. Description of the Prior Art
A monocrystalline semiconductor thin film formed on an insulating film has various advantages, as may be apparent in the case of silicon on sapphire (SOS). The monocrystalline semiconductor thin film can be physically isolated in an island shape or can be isolated using a dielectric so as to readily and completely achieve isolation of a semiconductor element formed in the monocrystalline semiconductor thin film. When an impurity is doped in the monocrystalline semiconductor thin film to a depth corresponding to an interface between the thin film and the insulating film, an area of a formed p-n junction becomes very small, thus decreasing a parasitic capacitance. As a result, high-speed operation of the element can be performed. Furthermore, when a MOS inverter is formed on the monocrystalline semiconductor thin film, the switching speed of the MOS inverter is increased since the substrate bias effect is not present. In addition, when the insulating film is a dielectric thin film formed on the monocrystalline semiconductor substrate, and the monocrystalline semiconductor thin film is connected to the monocrystalline semiconductor substrate through a groove or opening formed in the dielectric thin film, an electrical resistance is decreased. Thus, the above structure has more advantages than a structure in which a polycrystalline semiconductor thin film is formed on a dielectric thin film.
The structure in which a monocrystalline semiconductor thin film formed on an insulating film continues to a monocrystalline semiconductor substrate has been achieved by laser annealing to some extent. For example, such a structure is disclosed in Japanese Patent Disclosure (Kokai) No. 56-67923 (laid-open to the public on June 8, 1981) in which an insulating film having a groove is formed on a monocrystalline silicon substrate, a polycrystalline or amorphous semiconductor film is deposited to cover the insulating film including the groove, and the polycrystalline or amorphous film is scanned with a laser beam. The polycrystalline or amorphous semiconductor film is melted within a short period of time by radiation of laser beams. As a result, a monocrystalline thin film is grown on the insulating film using the monocrystalline semiconductor substrate as a seed crystal.
However, in the above technique utilizing recrystallization of the molten material, the molten material tends to gather on the insulating film. Thus, when the polycrystalline or amorphous semiconductor film is very thin, a continuous monocrystalline semiconductor thin film cannot be obtained. Further, since the polycrystalline or amorphous semiconductor thin film is first melted and then solidified, the obtained monocrystalline semiconductor thin film often will not have a smooth surface. Since a relatively high temperature is required to melt the polycrystalline or amorphous semiconductor thin film, atoms of a different type are diffused from the monocrystalline semiconductor substrate when the monocrystalline semiconductor substrate comprises a material different from that of the polycrystalline or amorphous semiconductor thin film. As a result, the obtained monocrystalline semiconductor thin film is contaminated.